1. Field of the Invention
The present invention relates to an optical signal detection circuit for detecting light in a light receiving unit, and more particularly to an optical signal detection circuit having a function to detect whether amplitude of intensity of received light reaches a predetermined value.
2. Related Background Art
Conventionally, one of the circuits for detecting whether amplitude of intensity of light received reaches a predetermined value, which is hereinafter called light-level detection circuit, is a circuit shown in FIG. 1.
An optical signal received by a light receiving unit is photoelectrically converted into an electric signal, e.g., shown in FIG. 2. This electric signal is applied to the base (node E) of a transistor 1 constituting a light-level detection circuit. The transistor 1 is turned on by this electric signal input, and charges are accumulated in a capacitor 2. With the accumulation of charges, the emitter voltage (node F) of the transistor 1 increases in accordance with the signal input as shown in FIG. 3. The capacitor 2 holds the peak value of the light input signal. The charge accumulation period is adjusted by a current source 3. A transistor 4 is biased to a fixed value by a reference voltage generating circuit 5, and the emitter voltage (node G) of the transistor 4 is set to a fixed voltage shown in FIG. 3. The emitter voltages of the transistors 1 and 4 are applied to a discrimination decision circuit 6. When the emitter voltage of the transistor 1 exceeds the emitter voltage of the transistor 4, which means that the input optical signal exceeds the reference voltage, the discrimination decision circuit 6 changes its output voltage (node H) to high level as shown in FIG. 4. In accordance with the high level of the output signal of the discrimination decision circuit 6, it is decided that the optical signal with a certain level has been applied to the light receiving unit. Note that a capacitor 7 and a current source 8 are to make the reference potential generating circuit symmetrical to the signal input circuit constituted with the capacitor 2 and the current source 3 to improve the property against noise.
In the regulation of communication system such as FDDI (Fiber Distributed Data Interface), SDH (Synchronous Digital Hierarchy) etc., it is required that an output signal which indicates that the light level resulting from the operation of the light-level detection circuit is above the predetermined value, which means that the input optical signal is stable, is set after a predetermined period from the signal input. Then, it is known from the output of the light-level detection circuit that the input optical signal has been stabilized, and information carried by the input optical signal is extracted from the input optical signal. For example, in the light-level detection circuit schematically shown in FIG. 5, the photoelectrically converted input optical signal is divided to a signal circuit comprising buffers 22 and 23 and a light-level detection circuit comprising buffers 24 and 25 and a delay element 26, through a buffer 21. The timing t.sub.1 of starting signal detection in the light-level detection circuit is delayed a predetermined delay time T.sub.D from the timing t.sub.0 of actual signal input in the signal circuit as shown in FIG. 5, which prevents the inconvenience case that the signal circuit does not rise although the signal detection circuit has been turned on.
In the conventional light-level detection circuit shown in FIG. 1, time at which the discrimination detection circuit 6 outputs the detection signal is delayed a time T from time at which the signal is applied to the transistor 1 shown in FIG. 4.